The present invention relates to a process for preparing pre-expanded particles of a thermoplastic resin and apparatuses used in the process. More particularly, the present invention relates, in preparing the pre-expanded particles where thermoplastic resin particles containing a volatile foaming agent which are dispersed in water under heating and an elevated pressure are released into a low-pressure area, to a process for recovering the foaming agent volatilized from the particles released into the low-pressure area; to a process for taking the pre-expanded particles out of the system with recovering the foaming agent; and to apparatuses used in the process.
Up to now a process for preparing pre-expanded particles of a thermoplastic resin where foaming agent containing particles are released into a low-pressure area after they are dispersed in water in a pressure vessel, for example, autoclave, and stirred under high pressure and high temperature is known as described in GE-OS (Offenlegungsschrift) 2,107,683, Japanese Patent Examined Publication No. 56-1344 and so on.
A description referring to preparing pre-expanded particles of a thermoplastic resin is indicated in the above-mentioned publications. However, any description referring to processes for recovering the foaming agent and to apparatuses applied to the processes in preparing pre-expanded particles in industrial scale is not indicated. The foaming agent generated in preparing pre-expanded particles is released into the air without recovery. As a result, that causes problems such as air pollution and the worsening of working environment, and economical problems such as increase in the material cost.